1. Field of the Invention
The present invention relates generally to an electrical oscillator circuit and, in particular, to a voltage-controlled oscillator with complementary transistors.
2. Description of the Related Art
Wireless radio communications systems transmit voice and other data between fixed transceivers and mobile radio communications terminals, or between two mobile terminals, via the propagation of radio frequency (RF) electromagnetic waves. It is essential to the functionality of such wireless communication systems that a stable and accurate generation of oscillating electrical signals be provided. The oscillating signals are supplied by oscillators, such as voltage controlled oscillators, connected in the communications system circuitry. Low phase noise oscillators are important components in wireless communication systems, particularly since phase noise critically affects the sensitivity of wireless transceivers. With advancements in sub-micron CMOS (Complementary Metal Oxide Semiconductor) technology, CMOS technology has become widely used for low cost and highly integrated RFICs (Radio Frequency Integrated Circuits).
Recently, single-chip transceivers that integrate both digital and RF circuits onto one chip using CMOS technology have been introduced. Among the building blocks in single-chip RFICs, design and implementation of a fully integrated low noise CMOS VCO (Voltage Controlled Oscillator) is known as a challenging circuit block because of the in-born limitations of silicon CMOS process technology. Most of the previously reported publications about CMOS VCOs describe the use of negative-Gm differential topology. In these publications, in order to optimize the phase noise performance, researchers stress the importance of layout issues such as active and passive device design, and design of the floor plan of the layout to reduce the side effects of the parasitic characteristics that are inherent in CMOS technology. See the publications by Thomas H. Lee, “The Design of CMOS Radio-Frequency Integrated Circuits”, Cambridge University Press, 1998; Chin-Ming Hung, Brian A. Floyd, Namkyu Park, and Kenneth K. O, “Fully Integrated 5.35 GHz CMOS VCOs and Prescalers,” IEEE Trans; and Donhee Ham and Ali Hajimiri, “Concepts and Methods in Optimization of Integrated LC VCOs,” IEEE ISSCC. Vol. 36, No. 6, June 2001.
In negative-Gm based sub-micron CMOS differential VCOs, the complementary structure shows a better performance than an NMOS-only (Negative-channel Metal Oxide Semiconductor) structure as result of the reduced hot carrier effect, better up/down swing symmetry, and higher transconductance of the constituting transistors.
Thus, by using low parasitic, simple, and high transconductance oscillator topology, there exists greater potential in the design of a low noise oscillator for use at high frequencies or with low power. Traditionally, a Colpitts oscillator, which has a simple oscillator core, has been the most favored topology for low phase noise. See the publication by Lawrence E. Larson, “Integrated Circuit Technology Options for RFICs-Present Status and Future Directions,” IEEE Journal of Solid State Circuits, Vol. 3, No. 3, March 1998. However, since the conventional Colpitts oscillator needs additional circuit elements for bias and buffer interfaces, its oscillation performances may be degraded by the parasitic effects in the circuit at high frequencies.
Due to the large number of passive and active components employed in conventional voltage-controlled oscillators (VCOs), which incur various parasitic effects, it used to be preferable to implement the circuitry in printed circuit boards with passive and active components mixed together rather than to integrate the circuit in silicon. However, the printed circuit board implementation causes high phase noise, resulting in serious problems in wireless communication, and occupies a large area that runs counter to the miniaturization trend of current wireless mobile stations and communication systems.
In addition to their use in wireless telecommunications, voltage controlled oscillators are used in many other types of devices, from computer mice, audio systems, telephones, computers, MP3 players, PDAs (Personal Digital Assistants), GPS (Global Positioning Satellite) devices to devices operating under the CDMA (Code Division Multiple Access), GSM (Global System for Mobile Communications), Bluetooth, WLAN (Wireless Local Area Network), and Zigbee standards.
In a typical wireless telephone or telecommunications device, a voltage controlled oscillator is provided as the local oscillator (LO) in the circuit and is connected to mixers in both the transmission and reception signal paths to provide the carrier signal for frequency synthesizing. If the LO (Local Oscillator) output contains phase noise, both the down-converted and up-converted signals are corrupted. Phase noise can be defined as random timing fluctuations in an oscillator period. Phase noise causes the ideal signal to be spread in the frequency spectrum, which results in frequency instability in the oscillator. Phase noise is generally specified in dBc/Hz at a given offset frequency from the particular carrier frequency. Therefore, phase noise can be found by measuring the ratio of the power spectral density (in a 1-Hz bandwidth) at a given offset frequency to the total power at the carrier frequency.
A typical voltage controlled oscillator has three functional portions of the circuit, namely a tank stage for selecting the oscillation frequency, an oscillation amplifier stage for amplifying the oscillation frequency signal selected by a tank stage, and a buffer amplifier stage for buffering and amplifying the final output signal. Many of the components in the traditional oscillator circuit have significant parasitic characteristics or parameters particularly at radio frequencies, resulting in poor performance of the circuit in a wireless communications system.
Traditionally, the Colpitts oscillator, which has a simple oscillator core, has been a favored topology for a low phase noise oscillator. However, since the conventional Colpitt oscillator needs additional circuit elements over and above those required for the oscillator core for bias and buffer interfaces, the oscillation performance of the Colpitts oscillator in practice may be degraded by the parasitic characteristics of these additional circuit elements at high frequencies. FIG. 1 shows the previous single-ended VCO implementation. The conventional VCO consists of a tank circuit 4 for selecting the oscillation frequency, an oscillation amplifier 2 for amplifying the selected oscillation frequency signal selected by the tank circuit 4, and a buffer amplifier 3 for buffering and amplifying the final output signal. In particular, the tank circuit 4 has an input for receiving a control voltage Vc. The control voltage is filtered by a capacitor C1 and is provided to an inductor L1 and a diode D1. Capacitors C2 and C3 are connected to an inductor L2 which then provides the frequency signal through capacitor C4. In the oscillator amplifier 2, the frequency signal is amplified by a transistor Q1 that has bias and buffer elements resistor R1 and resistor R2 and capacitors C5 and C6. The buffer amplifier 3 also has a transistor Q2 as well as a number of further bias and buffer elements resistors R3 and R4, capacitors C7, C8, C9, C10 and C11, and an inductor L3. The frequency signal is provided at the output of the voltage controlled amplifier at P0.